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WSN 81(2) (2017) 169-183 EISSN 2392-2192
Subcritical water extraction (SWE) of barley
(Hordeum vulgare) straw as method of antimicrobial and antioxidant additives production
Patrycja Sumińska1,*, Amandine Berton2, Wolfram Dietz3
1West Pomeranian University of Technology, Faculty of Food Sciences and Fisheries,
Center of Bioimmobilization and Innovative Packaging Materials, 35 Klemensa Janickiego Str., 71-270 Szczecin, Poland
2CELABOR, Avenue Du Parc 38, 4650 Herve, Belgium
3Papiertechnische Stiftung (PTS), Hessstrasse 134, Munich, Germany
*E-mail address: [email protected] *Phone: +48 91 449 6132
ABSTRACT
The effectiveness of Subcritical Water Extraction (SWE) process for extraction of antioxidants
and antimicrobials from barley straw (Hordeum vulgarae) was evaluated; also the impact of SWE
parameters on the quality of extracts has been assessed. Several methods have been employed: total
phenolic content and HPLC analysis (chemical structure of extracts), FRAP (Ferric ion Reduction
Antioxidant Power for antioxidant properties) and MIC (Minimal Inhibition Concentration for
antimicrobial properties). The temperature of process, time, solid ratio, flow rates, dynamic/static
mode, recirculation and washing of residues have an impact on the antioxidant and antimicrobial
activity of extracts. The polyphenols content in extract influences the quality of extracts; at least 20
polyphenolic compounds (including ferrulic acid, protocatechuic acid, chlorogenic acid, syringic acid,
coumaric acid, etc.) have been identified in extracts. The antioxidant and antimicrobial efficacy of
extracts differs depending on the origin of sample, temperature of extraction, duration of the process;
all these factors influences also polyphenols content. For all tested samples high antimicrobial activity
against Staphylococcus aureus and Escherichia coli has been observed. The storage of extracts at
room conditions decreases the antimicrobial efficacy. The SWE method is the “green” technology that
can be used to produce the mixture of active components in the form of extracts that can be applied in
food, cosmetics and pharmaceutical industry.
Keywords: Subcritical water extraction, barley straw, antimicrobial, antioxidant additives
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1. INTRODUCTION
Large amounts of biomass are produced industrially. Agri-food residues/by-products are
currently being disposed of in composting, they are used for feeding and/or littering, or are
incinerated with energy recovery. The necessity to shift from fossil to renewable resources
implies utilizing available biomass as efficiently as possible. Adding value to agricultural
leftovers is the most suitable for many applications, as no additional arable land is needed.
The high availability of raw materials, and high efficacy of the “green” process is the
guarantee of high suitability of such products for high-demand brands such as cosmetics, and
food industry.
Staphylococcus aureus and Escherichia coli are two opportunistic pathogens that are
responsible for moderate to severe and life-threatening infections, especially in
immunocompromised patients. There are an active substances e.g.: essential oils
[Kwiatkowski et.al., 2015], or plant extracts [Mizielińska et.al., 2015; Mizielińska et.al.
2017a, 2017b, 2017c] that can reduce or inhibit S. aureus or E. coli growth.
The subcritical water extraction (SWE) process is well known as the method that helps
to produce high-value extracts from bio-residues, such as hop waste (biomass from beer
production), fruit and vegetables residues (biomass from fruits and vegetables preserves), etc.;
such extracts are mostly applied in cosmetics. Due to their antioxidant properties and smells
they can be also used as scents and fragrances in bio-products. From the other side, the
European demand for natural antioxidants for the extension of the product shelf life has
boomed in the last ten years. This market is expected to outstrip that of their synthetic
equivalents. All natural antioxidants and antimicrobials such as essential oils, spices, and /or
natural plant extracts are also eco-friendly products in the comparison to synthetic ones.
Current market and social trends towards sustainable chemistry and environmentally oriented
manufacturing processes are supporting natural substances instead of synthetics. The “green”
certification of ingredients is becoming a requirement in the cosmetics (especially natural
cosmetics – market grows approx. 10% / year), nutraceuticals and food industry (especially,
eco-, “green” products). Barley (Hordeum vulgare) is cultivated worldwide and is one of the
most popular cereals. It can be used as a source of grains (application in food industry and as
feed for animals), straw (littering/bedding, feeding), and as a plant that can be used in
biorefinery (composting, bio-ethanol/production) and to produce energy (burning).
Additionally, as barley has higher content of cellulose than e.g. wheat straw (40% as opposed
to 30%) it has been explored as the potential feedstock in biofuel industry [Iroba et al., 2014]
and it can be also used in paper industry as the source of cellulose fibers. The barley plant
contains phenolic compounds (such as flavanals and tocopherols, phenolic acids, ferulic acids
and p-coumaric acids, syringic acid, vanilic acid, vanillin, syringaldehyde) [HoltekjØen et al.,
2006]. Bonoli and others (2004) and also other researchers have been studied the antioxidant
capacity and phenolic compounds content in barley [Liu and Yao, 2007; Pereira de Abreu et
al., 2012]. Some studies show that barley straw extracts can be applied as inhibitors of algae
growth, that is important in the management of water quality in water reservoirs [Pęczuła,
2013; Pilinger et al., 1994]. The subcritical water extraction (SWE) is the modern method to
obtain chemicals using green technology. SWE occurs when the temperature reaches at least
100 °C and the pressure is high enough to maintain the liquid state.
The unique properties of water are linked to its disproportionately high boiling point for
its mass, a high dielectric constant and high polarity. The aim of this study is to verify the
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suitability of barley straw as a source of added-value substances with antimicrobial /
antioxidant activity in the form of ready-to-use extracts. To obtain such extracts the ‘green’
method of subcritical water extraction (SWE) has been used. Among all new technologies
SWE, by reducing the usage of organic solvent, is a “green” solvent extraction method. It uses
pressurized water at higher temperature, and controlled pressure conditions to produce high-
condensed active extracts that can be applied in cosmetics, and food industry.
2. MATERIALS AND METHODS
2. 1. Raw material
Barley straw samples have been obtained from Polish and Belgian farmers, directly
from field, in the harvest season (from 2012 crop). They have grown in West Pomerania and
Wallonia region. The raw material has been stored at room conditions (23 +/-1 °C, 50-60%
relative humidity) prior to use. Before treatment (SWE) samples were ground in cutting mill
and sieved. Final sample had particle size approx. 4 mm.
2. 2. Subcritical water extraction (SWE)
The scheme of the process has been shown on Fig. 1.
Fig. 1. Scheme of the subcritical water extraction process.
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The equipment used for SWE was constructed in-house at the CELABOR, Belgium. All
connections/joints, fittings, tubing, valves and vessels were constructed of stainless steel to
resist corrosion. The extraction unit percolation is composed of 3 subunits: pumping section,
extraction sub-unit and recovery of extracts subunit. The equipment used for this study has
been designed for the extraction using solvents and gases. It consists a 5L-reaction chamber,
a heat exchanger, a recirculation pump and 2 solvent tanks (Fig. 2.).
Fig. 2. Scheme of installation for subcritical water extraction process (Celabor, Belgium)
Pressure (1·106 Pa) in the system was maintained at selected temperatures (120-160
°C)
during the whole process for all experiments by setting the pressure regulator. Nitrogen has
been flushed through the system in the reaction chamber to remove the air inside. 5l of
distilled water was heated, pressurized and pumped at a selected flow rate using a metering
pump. A 500 g sample (approximately 85% of dry matter) barley straw was uniformly packed
into a stainless steel basket in liquid/solid ratio 10:1. After the extraction the reactor has been
decompressed and cooled down with tap water to reach temperature below 30 °C.
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The extracts in liquid form have been freeze-dried. The final product in the form of
powder has been stored at room temperature.
2. 3. Effect of temperature and other parameters on extraction yield and extracts quality
Subcritical water extraction process is strictly dependent on temperature that is the key
parameter to increase the efficacy of the process. To investigate effect of temperature on the
yield and quality of extracts, extraction process was carried out using different extraction
temperature: 140, 150 and 160 °C, when the pressure was fixed at 1·106
Pa. The duration of
extraction was 15, 30 and 60 minutes. Solvent recirculation flow rates were: 0.5; 1 and 1.5
l/min. To verify the quality of extracts, also solid ratio, flow rates, extraction mode (dynamic
vs. static), and additional treatment such as washing out of residue were considered.
2. 4. Methods of quality of extracts assessment
2. 4. 1. Analysis of total phenolic content
The total phenolic content using gallic acid as a standard according to the Folin –
Ciocalteu method has been determined. The method is based on the ability of phenolic
compounds to reduce a mixture of phosphomolybdate and phosphotungstate salt in alkaline
medium.
The blue coloration occurred, and has been measured at 760 nm using
spectrophotometer (Germany). 20 mg of powdered extract was dissolved in 2 ml of water and
refilled with water to 25 ml of dispersion. 0.2 ml of extract dispersion was mixed with 0.8 ml
of Folin-Ciocalteu reagent, 1.2 ml of Na2CO3 at 200 g/l and 7.8 ml of water. After 2 hours of
incubation at room temperature the absorbance has been measured at 760 nm. Polyphenolic
content is expressed as g of gallic acid equivalents (GAE) per kg of dry extract or per kg of
raw material.
2. 4. 2. High Performance Liquid Chromatography (HPLC) and Ultra Performance
Liquid Chromatography with Mass Spectrometry (UPLC-MS) analysis of
polyphenols constituents
Simultaneously, the HPLC and UPLC-MS analysis has been performed.
Chromatographic separation of polyphenols was performed with following columns: Sepelco
ENVI-Chrom P (500 mg), Waters C18 (1g) and Waters Oasis HLB (200 mg) in HPLC system
(Knauer, Germany). Due to the fact, that gradient analysis has been performed, samples of 2%
extracts were dissolved in 2-phases titrant: solvent A – 2% of acetic acid and as solvent B -
50/50 vol./vol. acetonitryle/water with 0.5% of acetic acid. After overnight stirring, samples
were filtered using syringe filters with 0.2 µm PES membrane. The HPLC analysis was
performed using mixture of methanol/THF (25/75 vol./vol.). The concentrations of phenolic
acids in the sample were calculated from standard curves, from a plot of peak areas versus
concentrations for a series of standard solutions.
UPLC-MS analysis has been performed using RP18 column and a gradient using formic
acid buffer and acetonitryle. Polyphenols were quantified using external commercial standard
calibration curves following 2 specific mass transitions for each compound.
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2. 4. 3. The antioxidant properties test
A Ferric ion reducing antioxidant power (FRAP) assay was performed to evaluate the
antioxidant activity of extracts. The method is based on the reduction at low pH of a
colourless ferric complex (Fe3+
-tripyridyltriazine) to a blue-coloured ferrous complex (Fe2+
-
tripyridyltriazine) by the action of electron-donating antioxidants. The reduction was
monitored measuring the change of absorbance at 593 nm. The FRAP assay was performed
with the extracts solubilised in DMSO and diluted in water. FRAP reagent was prepared
directly before usage, by mixing one volume of 310 mg 2,4,6-tri[2-pyridyl]-s-triazine in 100
ml of an aqueous solution of HCl (40mM) with one volume of 540 mg FeCl3·6H2O in 100 ml
of acetate buffer (pH 3.6). This mixture was kept at 37 °C. Simultaneously, the calibration
curve using aqueous solution of FeSO4 (0.1 – 1 mM) has been prepared. Immediately, 60 µl
of blank FeSO4 standards and diluted extracts were mixed with 2 ml of FRAP reagent and the
solution was kept at 37 °C for exactly 4 min. Then the solution has been transferred to
a quartz cell and the absorbance was measured at 593 nm. The antioxidant activity of the
extracts is given as millimoles of Fe(II) equivalent/g of extract.
2. 4. 4. Antimicrobial properties verification
Minimal Inhibition Concentration (MIC) method was performed to verify the
antimicrobial efficacy of extracts. Two strains of bacteria have been tested: Escherichia coli
(Gram negative, DSMZ 1576) and Staphylococcus aureus (Gram positive, DSMZ 346). All
extracts in the form of powder were dispersed in distilled water in selected concentrations (0.5
– 15%). 250 µl of inoculum (1.5×108
CFU/ml, 24h of incubation at 37 °C) of specific
bacteria was placed in bullion in following ratio: 50% of double concentrated Tryptic Soy
Broth (TSB) (Merck, Germany) with 0.85% of NaCl and 50% selected concentration of
dispersed extract to get values of 0.5 – 10% concentration. Samples were incubated 24 hours
using laboratory shaker (300 RPM, rate per minute) at 37 °C. After incubation, serial dilutions
have been prepared, and inoculated on Petrie’s dishes. Escherichia coli was inoculated on
McConkey agar (Merck, Germany), Staphylococcus aureus on Tryptic Soy Agar (TSA)
(Merck, Germany) with 10% of NaCl. Plate cultures were incubated for 24 h at 37 °C, and
then colonies were counted. The antimicrobial activity has been given as log[CFU/ml]. All
measurements were done at least at triplicate.
3. RESULTS
3. 1. Subcritical Water Extraction process and effect of temperature and other
parameters on the quality of extracts and extraction yield
As shown on Fig. 3., temperature has a direct effect on the extraction yield, especially,
when the time of extraction is longer than 15 minutes. Time of process influences also the
efficacy. The effect is well visible, when the temperature of extraction is 160 °C.
Due to costs of process (heating) in further tests the raw materials have been extracted
no longer than 30 minutes – to optimize the duration of process vs. extraction yield. The
longer time of extraction is not recommended due to higher costs of process, even though the
extraction yield is the highest. To verify the quality of extracts, depending on time and
temperature of extraction, the antimicrobial and antioxidant assessment has been performed.
To screen the antioxidant and antimicrobial properties, as the first step, the extracts produced
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at 140-150 °C and 30 minutes of extraction have been tested. The extraction at lower
temperatures (below 140 °C) has lower extraction yield – even more than 100%. Solid ratio
and the flow rates have a side effect, but causes also increasing the efficiency of extraction
process. The increase of recirculation flow has direct effect on extraction yield; the higher
recirculation flow the higher extraction yield (even to 25% of increase, when the recirculation
flow rate has been increased from 0.5 to 1.5 L/min) has been observed.
Fig. 3. The effect of temperature and time of extraction on the extraction yield at a fixed
pressure of 1·106
Pa and fixed recirculation flow: 1L/min
The increase of temperature leads to a reduction of the polarity of the subcritical water,
and the rate of extraction increases with the temperature increase. Due to the fact, that at 160
°C hemicellulose and lignins start to be extracted, during the experiment temperature 160 °C
was the highest that has been tested.
Additionally, the temperature of extraction higher than 140 °C and longer time of
extraction promote formation of molecules of 5-hydroxymetylfurfural (5-HMF) and furfural
(due to Maillard reaction). To avoid the extraction of such molecules additional step of
recirculation of the solvent has been performed, that helped to limit the amount of 5-HMF and
furfural. The recirculation flow has a positive effect on extraction yield, antioxidant activity
and polyphenols content. Particularly, the increase of recirculation flow has direct effect on
extraction yield; the higher recirculation flow the higher extraction yield (even to 25% of
increase, when the recirculation flow rate has been increased from 0.5 to 1.5 L/min) has been
observed. Also the increase – approx. 10% - of antioxidant activity has been noticed, that is
linked to the increase of polyphenols content (5-6% of increase, when the recirculation flow
changes from 0.5 L/min to 1.5 L/min.).
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Two types of extraction mode have been also tested, dynamic vs static. The dynamic
mode was faster (15 min vs 60 min) and more efficient (yield: 17% by 15 min. vs 15.5% by
60 min.) also, the quality of extract was similar or higher (the content of specific polyphenols
obtained in dynamic mode e.g. coumaric acid 4339.05 mg/kg vs 3972.53 mg/kg – for extracts
obtained in static one).
Washing of residues after extraction has also positive impact on the efficacy of the
process. The extraction yield increases 25% after washing of residues. Also the quality of
extracts has been improved (the antioxidant activity and polyphenols content slightly
increases – approx. 6-7%).
All extracts have been evaluated after the extraction process in relation to antimicrobial
and antioxidant activity.
3. 2. Polyphenols content and antioxidant properties of extracts
In first screening analysis of barley straw extract (both from Belgium and Poland) more
than 20 polyphenols have been identified –and approximately quantified (Tab. 1).
Table 1. Screening and identification of polyphenols in barley straw extracts
(origin Poland and Belgium) (UPLC-MS analysis, Celabor, Belgium)
Polyphenols
Barley straw extract
(origin: Belgium)
Barley straw extract
(origin: Poland)
Rhamnetine
Luteoline
Linarine
Quercitrine
Oleuropeine
Hesperidine
Naringine
Dihydrokaempferol
Apigenine
Kaempferol
Naringenine
Quercetine
Rosmarinic acid
Phloridzine
Avicularine
Isoquercitrine
Rutine
Hyperoside
Ellagic acid
Ferrulic acid
Sinapic acid
x (+++)
x
x
x
x
x
x
x
x
x (+++)
x
x
x
x
x
x
x (+++)
x
x
x (+++)
x
x (+++)
x
x
x
x
x
x
x
x
x (+++)
x
x
x
x
x
x
x (+++)
x
x
x (+++)
x
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Coumaric acid
Chlorogenic acid
Vanilic acid
Hydroxybenzoic acid
Protocatechuic acid
Gallic acid
x (+++)
x
x
x
x
x
x (+++)
x
x
x
x
x
x - presence, +++ the highest amount
HPLC analysis confirmed the presence at least 20 polyphenols in barley straw extract
(origin Poland and Belgium). It has been also found that extraction temperature influences the
specific polyphenols content. For example, lower content of protocatechuic acid, caffeic acid,
chlorogenic acid, syringic acid has been noticed for sample that has been obtained at higher
temperature (160 °C), than for sample obtained at lower temperature (120 °C) (Fig. 4). In the
case of ferrulic acid its amount was higher for sample that has been obtained at higher
temperature. In both cases citric acid has been also identified. According to UPLC – MS
results also coumaric acid (approx. 0.148 % of dry mass), hyperosid, isoquercitin, and rutin
were found (less than 0.02% of dry mass).
Fig. 4. The specific polyphenols content in barley straw extracts produced at 120 and 160 °C
(origin of sample: Poland; fixed pressure and time) (HPLC analysis, all measurements were
done at least by triplicate)
The extraction temperature has an impact on antioxidant properties and polyphenols
content. The increase of temperature improves the antioxidant properties of extracts; this
tendency is clear visible for all samples. The effect is optimal, when the duration of extraction
process is 30 minutes. This tendency is weaker for samples that have been obtained during 15
and 60 minutes of extraction. The antioxidant properties are strictly related to polyphenols
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content (FOLIN analysis). In the case of 60 minutes extraction the highest amount of
extracted polyphenols have been noticed. This trend has been slightly dependent on extraction
temperature, when the extraction time was 60 minutes. When the extraction time was the
shortest (15 minutes), the highest amount of polyphenols has been noticed, especially, when
the extraction temperature was 150 0C. The clear tendency to increase the polyphenols content
with the increase of temperature has been observed in the case, when the extraction time was
30 minutes. The antioxidant capacity (Fig. 5a.) (and the polyphenols content – Fig. 5b.)
increased with the temperature of extraction and extraction duration.
a.
b.
Fig. 5. Effect of extraction temperature a. on antioxidant properties (FRAP) and b.
polyphenols content (FOLIN) values of barley straw extracts (sample origin: Belgium;
recirculation flow: 1L/min., fixed pressure)
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The highest yield in relation to antioxidant activity for 150 °C and 60 minutes has been
noticed. The higher temperature of extraction, the higher antioxidant activity and polyphenols
content in extracts has been observed; this tendency changes, when the extraction process is
longer than 30 minutes. Due to economic reasons (energy input for longer processes), for
further studies 15 and 30 minutes were considered
Some differences between samples of extracts (for samples obtained at 120 °C) from
Poland and Belgium have been noticed; FOLIN value for Polish sample (80 +/-2 mg/g) was
slightly higher than for sample from Belgium (65 +/- 2 [mg/g]), also FRAP value differed:
0.46 +/- 0.01 [mmol/g] for sample from Belgium and 0.60 +/- 0.01 [mmol/g] for sample from
Poland.
3. 3. Antimicrobial properties of extracts
The concentration of extracts in aqueous dispersions 5% w/w and higher has been found
as the most efficient one (Fig. 6.) to reduce the growth of Gram positive and Gram negative
bacteria. To reach the high antimicrobial effect, the concentration at least 7.5% w/w is
recommended, what is the high value in the comparison to synthetic substances/preservatives,
but taking into account the nature of extracts and the method of production, it is beneficial to
use natural extracts instead of synthetic preservatives.
Fig. 6. The minimal inhibition concentration for barley straw extracts (origin: Poland,
temperature of extraction: 120 °C; pressure: 1·106
Pa, recirculation flow: 1L/min)
(Staphylococcus aureus and Escherichia coli, incubation at 37 °C, 24h)
The antimicrobial efficacy against Staphylococcus aureus increases with the increase of
concentration; the antimicrobial effect started to be observable (al least 1 microbial log
reduction) when the concentration was 1% and higher. In the case of Escherichia coli, the
antimicrobial effect appeared, when the concentration was 1.5%. The microbial log reduction
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higher than 5log was observed for Escherichia coli, when the concentration of extract was
7.5%. The same concentration of barley straw extract showed 3log of microbial reduction for
Staphylococcus aureus. Also the origin of sample has an impact on antimicrobial properties
(Fig. 7). The barley straw extract from Poland has higher antimicrobial activity against
Escherichia coli (5 log microbial reduction) and Staphylococcus aureus (3 log microbial
reduction) than samples from Belgium (0.5 log microbial reduction – Escherichia coli; and
1.3 log microbial reduction – Staphylococcus aureus). The reason probably is that the
harvesting time differed in both countries; also some growing conditions matter (soil
composition, duration of days, weather conditions etc.).
Fig. 7. The comparison of antimicrobial activity of barley straw extract from Belgium and
from Poland (temperature of extraction: 120 °C; concentration of extracts 7.5% w/w,
bacteria strains: Staphylococcus aureus and Escherichia coli, incubation: 24h, 37 °C)
Also the extraction temperature influences the antimicrobial efficacy, both for Gram
positive (Staphylococcus aureus) and for Gram negative (Escherichia coli) bacteria strains.
Extracts obtained at 160 °C showed total microbial reduction at the 7.5% w/w/ concentration
– both against Gram positive (Staphylococcus aureus) and Gram negative bacteria
(Escherichia coli). The antimicrobial activity decreases to 0-1 log of microbial reduction after
11 months of storage at room conditions (23 °C, relative humidity RH 60-75%). This effect
has been observed for Gram positive and negative bacteria strains.
4. DISCUSSION
The limited research has been conducted to verify the suitability of barley straw as a
source of added-value substances such as antimicrobials and antioxidants that can be applied
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in other than agriculture sectors (food additives, cosmetics, pharmaceuticals). The analysis of
barley straw constituents in the relation to its antimicrobial and antioxidant properties is the
important issue in the sustainable world. A lot of studies (especially in 90ties) have been
performed to show the potential of barley straw extracts as algistatic and antialgal agents
[Pęczuła, 2013; Pilinger et al., 1994; Waybright et al., 2009]; its activity has been linked to
polyphenols content. They are the main inhibitory components and polyphenolic molecules
are responsible for activity profile [Waybright et al., 2009]. It has been also confirmed at this
study – the higher temperature of extraction was the important factor in the extraction of
polyphenols (Fig. 7), there was straight correlation between the amount of extracted
polyphenols and antioxidant and antimicrobial efficacy of extracts. It seems to be also the
correlation between the amount of Maillard reaction products such as 5-HMF and furfural and
the antimicrobial and antioxidant activity [Plaza et al., 2010]. The toxic effect of such
products is well known, but the amount of 5-HMF and furfural in all tested extracts stayed on
the acceptable level (less than 500 mg/kg of extract). The selected method – subcritical water
extraction - starts to be attractive for industry, a lot of various species of plant and agricultural
residues have been extracted, especially recently [He et al., 2012; Pushp Singh and Saldana,
2011; Rodriguez et al., 2006; Yogendra et al., 2011], but mostly the research work focused on
antioxidant efficacy of such extracts. Also the positive temperature effect has been observed
at many research studies [Aliakbarian et al., 2012; He et al., 2012; Pushp Singh and Saldana,
2011]; the extraction yields increased with temperature increase, due to increased solubility of
phenolics in water. The higher extraction temperature promotes degradation of phenolic
compounds, this process starts for barley straw at 160 °C, in the case of potato peels [Pushp
Singh and Saldana, 2011], the subcritical water extraction of such agricultural residues
promotes the de-composition and degradation of phenolic compounds at temperature 180 °C.
Such difference can be caused by difference in the structure of raw materials and availability
of phenolic compounds for molecules of water in subcritical state. For pomegranate seeds the
most optimal time of extraction was 30 minutes, and the most efficient temperature was 220
°C [He et al., 2012], also in this case the importance of extraction temperature was high. Plaza
et al. (2010) underlined the importance of temperature in relation to neo-forming bioactive
molecules (Maillard reaction) that are responsible for the antioxidant activity. It was also
confirmed in this study – the antimicrobial and antioxidant efficacy of extracts that have been
obtained at higher temperatures was also higher than in the case of extracts that were
produced at lower temperature. Barley straw is the important source of lignocellulosic
biomass; several methods of pre-treatment have been employed to assist in deconstruction and
disaggregation of the matrix, to get access to the cellulose and hemicellulose [Iroba et al.,
2014]. It was shown that such extraction residues could be used in paper industry [Dietz,
2015]. Also other parts of barley (than stem/straw like in this research work) have been tested
in relation to potential application as a source of antioxidants. For example barley husks have
been extracted to produce antioxidants that can be applied as the component of an antioxidant
active film for food package [Pereira de Abreu et al., 2012].
5. CONCLUSIONS
In the present study, it has been demonstrated that the use of SWE is valuable method to
produce added – value substances from barley straw. The process described here allows
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successfully producing and characterizing extracts that are enriched with polyphenols
fractions. The extraction yields are mostly dependent of temperature of extraction and other
extraction parameters. All extracts have demonstrated an antioxidant and antimicrobial
efficacy, their antimicrobial efficacy decreases in time when stored at room temperature.
Natural extracts obtained using subcritical water extraction method are promising source of
antimicrobials and antioxidants to be used in pharmaceuticals, cosmetics and food industry.
Moreover, the SWE process is the clean alternative that can be used in industrial scale.
Acknowledgement / Funding
The research work has been funded under the CORNET Programme by AiF and the German Federal Ministry
for Economic Affairs and Energy (BMWi), Germany, by Service Public de Wallonie (SPW), Belgium, and by
the National Centre for Science and Development (NCBiR), Poland. We would like to acknowledge this support,
and we also wish to thank the West Pomeranian University of Technology/Faculty of Food Sciences and
Fisheries, and CORNET Coordination Office and also the supporting industrial partners.
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( Received 12 July 2017; accepted 27 July 2017 )